The invention pertains to soil samplers, and in particular, to a soil sampler inserted into a pilot hole in the ground and positioned to a selected depth so as to volatize contaminants, and thereafter accumulate and transport contaminants to the surface for analysis.
It was not until 1975 that soil sampling was first attempted as a means to detect and trace ground contaminants. These early applications involved inserting pipes one to one-and-a-half feet into the ground to measure the evolution of both hydrocarbon and non-hydrocarbon gases. Since that time, the concept has taken two somewhat different directions in application; namely, in place passive soil gas sampling devices and mechanically inserted active soil gas sampling ground probes. The mechanically inserted sampling probes have involved large and small sampling approaches and several variations on sample extraction and analysis.
Numerous applications have involved auguring bore holes into the soil, placing a hollow breathing tube into the bore hole and backfilling with the cuttings. Sampling tips for obtaining soil gas samples range from glass tubing with flared openings to porous/sintered bronze cups and various perforated probe tips. These applications all demonstrated the same disadvantage; namely, that the auguring of the bore hole altered the soil gas composition and caused the loss of many of the gases of interest which changed relative as well as overall concentrations. The procedure required several days for the disturbance to equilibrate, which left numerous questions as a result of the variability of gaseous movements through the undisturbed soil. Furthermore, due to the absence of a reliable seal between the sampling point or probe shaft and the atmosphere, the dilution of the gaseous samples was found to cause false negatives and unreliable relative concentration results. Obviously, the above drawbacks detracted from the credibility of any analysis of these soil samples.
Alternative techniques intended to correct the two major deficiencies of the previous method were developed and took the form of mechanically inserting probes with sacrificial penetration tips or perforated well points to the desired depth. The expense involved with sacrificial penetration tips has been a drawback to that approach. This procedure minimized the disturbance of the soil matrix, caused no significant release of soil gases, and provided an improved natural seal between the sampling point and probe shaft and the surface atmosphere. Variations of this type of "breathing" soil probe have been widely applied. The principal limitation of the perforated well-point method was the blinding or clogging of the sample ports with soil during the insertion process. This led to the development of several types of closed well-points which were opened mechanically upon reaching the desired depth by extracting the insertion casing.
Early researchers found the mechanical insertion method to be preferable to the augured insertion technique and more reliable in obtaining accurate, reproducible soil gas samples. However, some of these commercially available well-points continue to experience problems in opening as a result of the high compression experienced during the insertion process. The principal limitation of this process was the relatively high volume of gas within the interior of the probe that needed to be extracted prior to obtaining a representative soil gas sample. Also, dilution of the sample becomes a problem when the volume of the gas within the probe is relatively large. Structural demands required that the probe shafts be relatively heavy to withstand the rigors of the insertion process which limited the freedom to physically down-size the casing shafts. This limitation was overcome by redesigning the well-points to allow for the attachment of laboratory tubing between the well-point and the surface where the sample is withdrawn. The benefit of this process was that a smaller sample volume was necessary to purge the system, and consequently, a sample more representative of the interstitial gases was obtained at the surface sampling point.
The various methods of manually and mechanically inserting the probes range from manual drop-hammers, to hydraulic and mechanical impact hammers, to hydraulic penetration devices. The insertion of the sample point by driving the unit with an impact/percussion hammer on the upper end of the connecting shaft results in greater by-passing of diluting atmospheric gases during sampling. The cause of this problem is the compacting of the soil which contacts the probe shaft during insertion by the vertical impact/force being converted to lateral motion by the combining forces of the impact and the counterpoising resistance to penetration and friction.
Sample extraction techniques have ranged from passive, atmospheric movement of the soil gases to the surface, to low and high vacuum extraction methods. The passive process was based on the natural diffusion of gases from higher concentrations in the soil to the sampling ports in the well-point. Since this process was based on the natural concentration gradient, researchers often found that tight, clayey soils were too restrictive to the natural movement of soil gases. This technique also required considerable time for soil gases to collect within the sampling probe for final drawing for analysis. Therefore, it was limited to relatively high concentrations of volatile contaminants.
The application of low vacuum to the sampling probe was the first attempt to address this limitation. This reduced the time required for the accumulation of soil gases within the sampling probe but did not materially affect the migration of gases through tight soils to the well-point. High vacuum extraction processes were later developed to increase extraction efficiency. However, this technique required the development of gas cleaning systems at the surface since ground water and soil particles were often drawn through the probe. Also, structural improvements to the system were necessary to accommodate the larger pressure differential. A significant additional advantage of the high vacuum systems was the reduction of the partial gas pressure in the sampling zone of the soil causing heavier and more complex molecules to volatilize at the reduced pressures.
Analytical methods used to process the extracted sample have included colormetric gas tubes, gas bag samplers, carbon adsorbing tubes, combustible gas indicators and gas chromatographs. Portable, laboratory quality flame- and photo-ionization detector/gas chromatographs have provided a mechanism whereby the extracted soil gas samples can be separated into their various components and can be analyzed on-site. Gas chromatography has utilized both flame-ionization detectors and photo-ionization detectors, as well as electron capture and far-ultraviolet detectors. The most common processes deliver the sample to the analytical device by the use of gas tight syringes to extract the gas sample, and most recently by extracting gas samples directly from the vacuum pump exit stream. This second procedure is more expeditious but requires care so as not to over pressurize the analytical instrument with exit gases from the vacuum pump. Also, accuracy in measuring the flow of a moving stream as well as incorporating effective gas cleaning devices for removing moisture and soil particles are necessary in order to obtain a representative sample for analysis.
Certain commercially available, closable well points have exhibited a tendency to jam with soil under repeated opening and closing while in the subsurface soil regime. The repeated opening and closing of the well points which is necessary when readings are taken during insertion can result in either the blinding or clogging of the sampling ports or the jamming of the protecting shroud itself restricting the ability to open the point when reaching the succeeding test elevation.
Chemical factors affecting the soil vapor assessment process include the variations in vapor pressures and boiling points of the compounds of interest themselves, and the appropriateness of the detection and analytical devices employed in relation to the contaminant compounds. The relationship between the various soil conditions can be characterized by the following formula. EQU C.sub.g /C.sub.t =1/((P.sub.g K.sub.oc f.sub.oc /K.sub.h)+(O/K.sub.h +a))
where:
C.sub.g /C.sub.t =Relative Vapor Concentration. [(mg/cm.sup.3 .cndot. air)/(mg/cm.sup.3 .cndot. soil)] PA1 P=Bulk Density (g/cm.sup.3) PA1 K.sub.oc =Organic Carbon-Water Partition Coefficient (cm.sup.3 /g) PA1 f.sub.oc =Fraction of organic carbon content (g/g) PA1 K.sub.h =Henry's Constant [unitless ratio] PA1 O=Volummetric Moisture Content (cm.sup.3 /cm.sup.3) PA1 a=Volummetric Air Content (cm.sup.3 /cm.sup.3) PA1 C.sub.v =concentration in vapor phase (i.e., the vapor in soil interstices) (mmHg) PA1 X=mole fraction of compound in solution PA1 C.degree..sub.v =concentration in vapor phase above pure liquid (i.e., vapor pressure) of the component of interest (mmHg) PA1 C.sub.v =concentration in vapor phase PA1 K.sub.H =Henry's Law coefficient PA1 C.sub.w =concentration in aqueous phase
Research into the behavior of wet and dry gaseous compounds and the laws affecting gases have provided a number of constants and predictable reactions for the contaminant compounds commonly found in soil matrices. Soil vapor contaminant assessment is possible since vapors indicative of contamination resulting from volatile organic compounds are present within the voids of the soil interstices. These contaminant vapors evaporate from contaminated water or result from non-aqueous phase liquids (NAPL) released in the vadose (unsaturated) zone of the soil.
Research has shown that both Henry's and Raoult's Laws are important in understanding the equilibrium of contaminant concentrations resulting from volatilization of aqueous and non-aqueous phase liquids. Raoult's law describes the equilibrium vapor phase concentrations above a dissolved contaminant in higher concentrations. EQU Raoult's Law: C.sub.v =X C.degree..sub.v
where:
Henry's Law describes the equilibrium vapor phase concentration above water containing an organic solute at low concentrations. EQU Henry's Law: C.sub.v =K.sub.h C.sub.w
where:
Based on Raoult's and Henry's laws and the vapor pressures of various organic compounds, it is clear that not all contaminants can be detected using soil vapor assessment techniques. The compounds of interest must be sufficiently volatile to enter the vapor phase in sufficiently high concentrations to be extracted and detected and must have an adequately low aqueous solubility so that they do not remain in aqueous solution. Currently, soil vapor analysis are limited by the vapor pressure and Henry's Law constant (i.e., vapor pressure and aqueous solubility) for the compounds of interest. Generally, for the process to be reliable, the contaminant compounds should have a vapor pressure of at least 1.5 mmHg at 25.degree. C. and a Henry's Law constant of at least 0.1 kPam.sup.3 per mole. The list set out below is a partial listing of compounds that are amenable for soil vapor assessments based on current procedures.
______________________________________ VAPOR HENRY'S LAW PRESSURE CONSTANT (mmHg) (kPa M3/mole) ______________________________________ Benzene 95 0.6 Bromoform 5 0.06 Bromomethane 1300 0.5 Carbon Tetrachloride 15 2.0 Chlorobenzene 12 0.4 Cloroethane 700 0.2 Chloroform 190 0.4 Chloromethane 3750 1.0 1,2 Dichlorobenzene 1.5 0.2 1,3 Dichlorobenzene 2 0.4 1,1 Dichloroethene 500 15.0 1,2 Dichloroethene 700 0.1 1,1 Dichloroethene 200 0.6 trans-1,2 300 0.6 Dichloroethene 1,2 Dichloropropane 50 0.4 cis-1,3 40 0.2 Dichloropropene Ethylbenzene 9 0.8 Methylene Chloride 350 0.3 1,1,2,2 5 0.05 Tetrachloroethane Tetracholorethene 15 2.0 Toluene 28 0.7 1,1,1 100 3.0 Trichloroethane 1,1,2 25 0.1 Trichloroethane Trichloroethene 50 0.9 Vinyl Chloride 2200 50.0 o-xylene 6 0.5 m-xylene 8 0.7 p-xylene 8 0.7 ______________________________________
Several researchers have attempted to change the subsurface conditions in an attempt to expand the range of compounds amenable to the soil vapor assessment process. One researcher reported that the detectability of organic compounds depends upon their ability to volatilize into the porous spaces of the soil and that most contaminants volatize either directly from the soil or from dissolved aqueous phase. Since the temperature of the subsurface soil below one meter reflects the mean ground water temperature of 65.degree. F. or less, normal low vacuum gas extraction is generally limited to those compounds having standard condition boiling points of less than 150.degree. F. and vapor pressures greater than 0.002 atmospheres.
The capacity to detect organic compounds by soil vapor techniques depends upon contaminant volatility. Soil vapor concentrations are related to two governing systems: water phase and non-aqueous product phase. Raoult's and Henry's laws are commonly used to understand equilibrium vapor concentrations governing the volatilization from liquids. Therefore, the ability to manipulate or change the variables in these equations will allow for expansion of the range of compounds suitable and detectable by soil vapor extraction techniques. The theory of the interaction between soil temperature and the role of pressure has not been fully developed. However, research and experience have empirically established without question that elevating sub-surface soil temperature and reducing the local atmospheric pressures will result in a broadened spectrum of extractable compounds. It has also been established that the release of volatile gases form a soil matrix is influenced by the moisture content of the soil. Gases adsorb more strongly to dry soil particles and conversely will desorb to a greater extent from moist particle surfaces.
Experience with in situ vacuum extraction has shown that moist soils are dried by the vacuum extraction process. Some decrease in the release of volatile organic compounds will result due to enhanced adsorption between the hydrocarbon molecules and the soil particle surfaces due to drying as extraction progresses. This increased adherence can be reversed or avoided by maintaining adequate soil moisture during the extraction process. This phenomenon is mathematically described by the Relative Vapor Concentration Formula. By applying and moisture heat to the sub-surface soil conditions, ensuring that drying of the soil matrix does not occur, and reducing the ambient pressure in the zone of extraction, compounds with boiling points of up to 400.degree. F. and 0.007 atmospheres vapor pressure can be suitable for soil vapor analysis.